Zeolitic materials, both natural and synthetic, have been demonstrated to have catalytic properties for various types of hydrocarbon conversion. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline structure as determined by X-ray diffraction, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. These cavities and pores are uniform in size within a specific zeolitic material. These materials have become known as molecular sieves because the dimensions of these pores can accept for adsorption molecules of certain dimensions and reject those of larger dimensions. Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline silicates. These silicates can be described as rigid three-dimensional frameworks of SiO.sub.4 and Periodic Table Group IIIA element oxide, e.g., AlO.sub.4, in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total Group IIIA element, e.g., aluminum, and silicon atoms to oxygen atoms is 1:2. The electrovalence of the tetrahedra containing the Group IIIA element, e.g., aluminum, is balanced by the inclusion in the crystal of a cation, e.g., an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of the Group IIA element, e.g., aluminum, to the number of various cations, such as Ca/2, Sr/2, Na, K or Li, is equal to unity. One type of cation may be exchanged either entirely or partially with another type of cation utilizing conventional ion exchange techniques. By means of cation exchange, it has been possible to vary the properties of a given silicate by suitable selection of the cation. The spaces between the tetrahedra are occupied by molecules of water prior to dehydration.
Prior art techniques have resulted in the formation of a great variety of synthetic zeolites. Many of these zeolites have come to be designated by letter or other convenient symbols, as illustrated by zeolite Z (U.S. Pat. No. 2,882,243); zeolite X (U.S. Pat. No. 2,882,244); zeolite Y (U.S. Pat. No. 3,130,007); zeolite ZK-5 (U.S. Pat. No. 3,247,195); zeolite ZK-4 (U.S. Pat. No. 3,314,752); zeolite ZSM-5 (U.S. Pat. No. 3,702,886); zeolite ZSM-11 (U.S. Pat. No. 3,709,979); zeolite ZSM-12 (U.S. Pat. No. 3,832,449); zeolite ZSM-20 (U.S. Pat. No. 3,972,983); zeolite ZSM-35 (U.S. Pat. No. 4,016,245); and zeolite ZSM-23 (U.S. Pat. No. 4,076,842), to name a mere few.
The SiO.sub.2 /Al.sub.2 O.sub.3 ratio of a given zeolite is often variable. For example, zeolite X can be synthesized with SiO.sub.2 /Al.sub.2 O.sub.3 ratios of from 2 to 3; zeolite Y, from 3 to about 6. In some zeolites, the upper limit of the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is unbounded. ZSM-5 is one such example wherein the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is at least 5 and up to the limits of present analytical measurement techniques. U.S. Pat. No. 3,941,871 (Re. 29,948) discloses a porous crystalline silicate made from a reaction mixture containing no deliberately added alumina in the recipe and exhibiting the X-ray diffraction pattern characteristic of ZSM-5. U.S. Pat. Nos. 4,061,724, 4,073,865 and 4,104,294 describe crystalline silicates of varying alumina and metal content.
It is generally known that the properties of zeolites can be influenced by changing the structural silica-to-alumina mole ratios. In synthesizing the zeolite the ratio can be varied by altering the relative amounts of the silica and alumina-containing precursor materials. For example, increasing the silica relative to the alumina usually results in a higher silica product. However, after a certain silica-to-alumina mole ratio is achieved, proportionally increasing the silica content of the reactants does not necessarily effect the silica-to-alumina mole ratio of the final product and can even hinder the formation of the desired final product.
Zeolite Beta is a known zeolite which is described in U.S. Pat. Nos. 3,308,069 and RE 28,341 both to Wadlinger, and reference is made to these patents for a general description of zeolite Beta. The zeolite Beta of Wadlinger is described as having a silica-to-alumina ratio going from 10 to 100 and possibly as high as 150.
Highly silicious zeolite Beta described as having silica-to-alumina ratios within the range of 20-1000 is disclosed in Valyocsik et al, U.S. Pat. No. 4,923,690. To achieve the high silica-to-alumina ratio the zeolite is only partly crystallized. As the zeolite becomes more fully crystalline, the silica-to-alumina ratio decreases. This is demonstrated in the examples which show achievement of highly silicious zeolite Beta at between 30 and 50% crystallinity. It would be desirable to achieve a highly silicious zeolite Beta which is fully crystalline.
The description of the zeolite Beta of the Wadlinger patents is silent as to the crystallite size. Typically, however, the zeolite Beta produced by the Wadlinger method is a small crystal zeolite Beta having a crystal size ranging from 0.01 to 0.05 microns. For certain applications, large crystal zeolites have been found to possess distinct advantages over the smaller crystal zeolites.
Larger crystal zeolites are known to provide longer diffusion path lengths which can be used to modify catalytic reactions. By way of illustration only, in the medium pore zeolite ZSM-5, manipulating crystal size in order to change the selectivity of the catalyst has been described. A unique shape selective characteristic of ZSM-5 is the para-selectivity in toluene disproportionation and aromatics alkylation reactions. Increasing the size of the crystal, thereby lengthening the diffusion path, is just one way of achieving a high para-selectivity. The product selectivity occurs because an increase in the diffusion constraints is imposed on the bulkier, slower diffusing o- and m- isomers which reduces the production of these isomers and increases the yield of the para-isomer. N.Y. Chen et al, Shape Selective Catalysis in Industrial Applications, p.p. 51 (Marcel Dekker, Inc New York 1989) and N.Y. Chen et al, Industrial Application of Shape Selective Catalysts, p.p. 196 (Catal. Rev. Sci. Eng. 28 (2&3) 1986). Obtaining high selectivities in zeolite ZSM-5 by increasing the crystal size is described in U.S. Pat. No. 4,517,402 which is incorporated herein by reference. In U.S. Pat. No. 4,828,679 it is revealed that large crystal ZSM-5 type zeolites have improved octane gain and total motor fuel yield as well as improved steam stability. U.S. Pat. No. 4,650,656 describes a large crystal ZSM-5 which is synthesized by controlling the reaction conditions such as the rate of addition of the organics, the temperature, pH and the degree of agitation of the crystallization media. The application of an external gravitational force during the synthesis of silicalite has been described as a means for producing a large crystal zeolite in D. T. Hayhurst et al, "Effect of Gravity on Silicalite Crystallization" in Zeolite Synthesis p.p. 233 (M. L. Occelli Ed. American Chemical Society 1956). In J. F. Charnell, "Gel Growth of Large Crystals of Sodium A and Sodium X Zeolites", Jour. Crystal Growth 8, pp. 291-294, (North Holland Publishing Co., 1971), a method of synthesizing large crystal zeolite A and zeolite X is described in which, as the only organic reactant, triethanolamine is incorporated into the reaction mixture. A review of these publications reveals that a significant amount of attention has been directed to synthesizing large crystal zeolites yet none of the publications point to a consistent method for producing the large crystals. The crystal size of zeolite Beta was generally related to the silica-to-alumina mole ratio, the highly silicious zeolite Beta corresponding to a larger crystal size and the lower silica-to-alumina ratio corresponding to a smaller crystal size. Techniques for synthesizing a large crystal zeolite Beta covering a broad range of silica-to-alumina mole ratios, including the high as well as the low silica-to-alumina ratios, would be desirable.